[Antennas] Mosley TA-33 antenna/coax problem

Tue, 1 Jul 2003 15:08:54 -0400

has anyone checked the quality of the coax, (probably old and probably has
water
incursion) ?  the coax may have already turned the deadly GREEN inside ?
ltr,
73
wb4izr
barry
----- Original Message ----- 
From: "James Duffer" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 01, 2003 1:16 PM
Subject: [Antennas] Varying feedline length (was Mosley TA-33)


> It was written
>
> "you cannot change the vswr of the antenna by changing the feedline
> length...what is happening most likely is you are bringing the feedline
> length close to a multiple of 1/2 wave length at the operating
> frequencey and therefore getting a mirror-image of the vswr at the
feedpoint
> showing at the meter in the shack...don't forget the correct place to
check
> the
> vswr is at the the antenna itself and not in the shack but sometimes this
is
> not
> physically possible so the best thing to do is get an exact image in the
> shack end by having the feedline in multiples of 1/2 wavelength to
> enable this...this is an electrical length not physical length length and
> the
> formula to calculate this is by 468/F xVF where V is the Velocity Factor
> of the cable in use and F is the Frequency in use"
>
> Very true.  However, as was written by another, he was advised by a
> manufacturer to change the lenght of his coaxial feed.  Now why would he
> tell him that?  And what about the success he obtained when he DID change
> the feed length?  It is knows fact that a certain fractional electrical
> wavelength lenght of coaxial cable can and does transform impedances, a
> quarter wave can transform a short at one end to appear as an "open" at
the
> opposite, now what do you think appears at various points in between?
>
> My advice is to go ahead and try what the manufacturer recommended and if
it
> don't work try something else.
>
> 73, Jim, wd4air
>
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